1. Field of the Invention:
The present invention relates to a tone control circuit and more particularly a circuit adapted for adjusting the tone of a sound frequency signal, more especially a musical signal, in a sound signal diffuser such as a radio receiver, a television set, a recorded disk apparatus or a magnetic recording diffusion apparatus.
2. Description of the Prior Art:
In these different apparatus, it is currently provided for the user to be able to select the volume in different ranges of the frequency band so as to favor for example the bass (low frequency) or treble (high frequency). This adjustment is designed so as to adapt the sound diffusion to the particular musical tastes of the user and/or to correct the defects existing in the recorded or transmitted sound information.
Among the numerous circuit structures for obtaining this result, we frequently find at present circuit structures of the type shown in FIG. 1, in which a sound signal arriving at an input 1 is fed into three filters 2, 3 and 4, respectively a high-pass filter, a band-pass filter and a low-pass filter. These filters are disposed in parallel and their action on the incident signal is greater or smaller depending on the adjustment of the electronic potentiometers 5, 6 and 7 in series with each of these filters. The output signals of the electronic potentiometers are fed through a mixer circuit 8 to other circuits 9 required for shaping and suitably amplifying the electric signals for their transformation into sound form by means of a transducer such as a loud-speaker.
Nowadays, the aim is to insert the greater number of electric components of a given assembly on the same wafer in the form of an integrated circuit. That is generally not possible for filter elements which comprise capacitors and resistors of a well-defined value. Thus, in a circuit such as that in FIG. 1, the filters 2, 3 and 4 would be disposed outside the integrated circuit containing the elements surrounded by the broken line 10 and three terminals 11, 12 and 13 would have to be provided for access to the inputs of the electronic potentiometers and an output terminal 14.
As is well known, it would be advantageous in order to obtain a reduction in noise to dispose the different filters of each of the channels after the electronic potentiometer of the corresponding channel and not in front, so as to eliminate the noise produced by this electronic potentiometer in the band rejected by the filter. This arrangement is currently applied in circuits constructed from discrete elements but not in circuits constructed in integrated circuit form. In fact, as is shown in FIG. 2, in which the same elements are designated by the same references as in FIG. 1, instead of four terminals 11, 12, 13 and 14 for access to the integrated circuit 10, it would be necessary to provide eight access terminals, namely an input terminal 20, three output terminals 21, 22 and 23 for each of the electronic potentiometers, three input terminals 24, 25 and 26 feeding into a mixer or adder 8 and an output terminal 14 for this adder.
Now, it happens that one of the most important cost items in an integrated circuit resides in the number of its access terminals, i.e. in the number of its connection pins. It is also one of the most important sources of defects in integrated circuits. To obtain an improvement in the signal/noise ratio it is then out of the question to envisage providing four additional access terminals for the integrated circuit.
FIGS. 3A, 3B and 3C show by way of example particularly simple passive filters able to serve respectively as high-pass, band-pass and low-pass filters. The high-pass filter of FIG. 3A comprises a series element formed of a capacitor 31 and a parallel element formed by a resistor 32. The band-pass filter of FIG. 3B comprises a series element formed by a capacitor 33 and a resistor 34 in series and a parallel element formed by a resistor 35 and a capacitor 36 in parallel. The low-pass filter of FIG. 3C comprises a series element formed by a resistor 37 and a parallel element formed by a capacitor 38. Thus, the simplest filter assembly corresponding to the three filters 2, 3 and 4 of FIG. 2 comprises eight discrete components, i.e. four capacitors and four resistors. Such a number of discrete components would take up considerable space, in practice more than the integrated circuit comprising the different amplifiers 5, 6 and 7.